This invention relates generally to phosphorus fertilization of plants. More particularly, the invention relates to a composition and a process for providing continuous release phosphate fertilizer involving mineral phosphate solubilizing (Mps.sup.+) bacteria or other microorganisms and rock phosphate ore, wherein the ore is solubilized and released into the soil by a solubilizing agent produced by the bacteria or other microorganisms.
Phosphorus is essential for virtually all major metabolic processes in plant growth and development. It is considered a macronutrient because it makes a relatively high percentage contribution to plant biomass. However, phosphorus is one of the least soluble mineral nutrient ions in soil. In fact, the level of phosphorus in the solution phase of naturally occurring soils is often below that of many micronutrients. E. Epstein, Mineral Nutrition of Plants 44 (1972); M. Fried & H. Broeshart, The Soil-Plant System in Relation to Inorganic Mineral Nutrition 545 (1967). The low availability of phosphorus makes it the limiting element for plant growth in natural ecosystems. P. Ozanne, Phosphate Nutrition of Plants--A General Treatise, in The Role of Phosphorus in Agriculture 559-85 (F. E. Khasawneh et al. eds., 1980). Not surprisingly, most higher plants have highly efficient mechanisms for absorbing phosphorus from soil. However, maximum agronomic productivity is achieved only with addition of phosphorus fertilizer.
Most phosphorus fertilizers in use today are made by processes involving mining and chemical processing of insoluble mineral phosphate ore, primarily fluorapatite. R. Young & C. Davis, Phosphate Fertilizers and Process Technology, in The Role of Phosphorus in Agriculture 195-225 (F. Khasawneh et al. eds., 1980); M. Fried & H. Broeshart, The Soil-Plant System in Relation to Inorganic Mineral Nutrition 545 (1967); W. Horwitz ed., Methods of Analysis of the Association of Official Agricultural and Food Chemists--Phosphorus 9-14 (AOAC, 13th ed. 1980). Chemical conversion of mineral phosphate ore into soluble orthophosphate is an energy intensive process involving treatment of the ore with sulfuric acid at high temperatures. This results in almost complete solubilization of the ore, but undesirable contaminants are released into gas streams, by-product streams, and soluble phosphate products. Thus, pollution abatement is a substantial cost to the phosphate fertilizer industry. Further, relatively high grade ore is required, lower grade ore being avoided in the mining process or rejected as tailings, thus driving up costs even more. Methods of selective, low temperature conversion of mineral phosphate ore to soluble phosphate would alleviate pollution, increase the effective ore reserves, and decrease production costs.
U.S. Pat. No. 5,256,544 to Rogers et al. describes an industrial scale continuous bioprocess for solubilizing rock phosphate ore by microbial action. The method involves forming an aqueous mixture of phosphate solubilizing microorganisms and phosphate ore particles of an appropriate size and maintaining the mixture under conditions whereby the phosphate ore particles are solubilized by a solubilizing agent produced and released by the microorganisms. The mixture is then fractionated into an aqueous fraction containing the soluble phosphate and a slurry fraction containing undissolved solids. The soluble phosphate is removed from the aqueous fraction, and the microorganisms present in the aqueous fraction are then recycled together with the undissolved solids of the slurry fraction to continue the solubilization and separation process.
Fertilizers produced from rock phosphate ore are largely soluble in aqueous solution and include ammonium, potassium, and sodium orthophosphates, several forms based on monocalcium phosphate (super, triple super, and concentrated superphosphate), ammonium and urea-ammonium (poly) phosphates, and various phosphate solutions. These fertilizers are applied to the soil by various cultural methods to maintain the amount of soluble phosphate above a level that gives &gt;90% of maximum yield. P. Ozanne, Phosphate Nutrition of Plants--A General Treatise, in The Role of Phosphorus in Agriculture 559-85 (F. E. Khasawneh et al. eds., 1980).
Agronomic rates of application of phosphorus fertilizer are usually well above what would be needed under ideal conditions. While competition with soil microorganisms diminishes fertilizer efficiency somewhat, a major role in the inefficient use of phosphorus fertilizer in agriculture is retention or fixation of phosphate in insoluble mineral complexes. In some soils, up to 75% of applied phosphorus may be reprecipitated into insoluble mineral forms, requiring application of up to 4 times the phosphorus needed by the crop to compensate for this unavailability.
Phosphorus is almost immobile in the soil, thus agronomically useful phosphorus fertilization must occur at the root/soil interface, known as the rhizosphere. The prevailing agronomic philosophy has been to maintain the levels of ionic phosphate in the bulk soil solution above the critical level that will provide adequate nutrition to plants. An alternative to this approach involves the regulation of phosphate availability within the rhizosphere. A microbial agent acting to solubilize mineral phosphates would be an important component of this system.
The presence of mineral phosphate solubilizing microorganisms in the rhizosphere has often been suggested. F. Gerretsen, 1 Plant and Soil 51-81 (1948); W. Sackett et al., 20 Central bl. Bakteriol. 688-703 (1908). Recently, the symbiotic plant/fungus associations known as mycorrhizae have received attention for improving phosphorus mineral nutrition. P. Tinker, The Role of the Rhizosphere in Phosphorus Uptake by Plants, in The Role of Phosphorus in Agriculture 617-47 (F. Khasawneh et al. eds., 1980). The possibility that rhizosphere bacteria may act to make phosphate available has also been explored. In F. Gerretsen, 1 Plant and Soil 51 (1948), it was suggested that rhizosphere microorganisms play a major role in providing plants with soluble phosphate. In J. Sperber, 180 Nature 994 (1957), lactic, glycolic, fumaric, and succinic acids were identified from cultures of putative mineral phosphate solubilizing bacteria and in soil samples. Digestion zones of from 1 to 10 mm in width around colonies were shown in 84 of 291 bacterial isolates on insoluble phosphate agar. In all cases, solubilization was accompanied by decreases in pH to as low as pH 4.5. These types of bacteria were later shown to be more prevalent in the rhizosphere than in nearby soil. J. Sperber, 9 Aust. J. Agric. Res. 778 (1958). H. Louw and D. Webley, 22 J. Appl. Bact. 216 (1959), obtained over 100 bacterial isolates capable of dissolving insoluble phosphate on agar plates and identified lactic and 2-ketogluconic acids as potential solubilizing substances, but could not correlate the occurrence of these bacteria with the rhizosphere. R. Duff & D. Webley, Chem and Ind. 1376 (Oct. 31, 1959), proposed that 2-ketogluconic acid produced by bacteria plays an important role in solubilizing phosphate in soil.
"Biosuper," a mixture of mineral phosphate, granulated sulfur, and sulfur oxidizing bacteria such as Thiobacillus spp., has been reported to provide effective phosphorus fertilization under some conditions. K. Raghu & I. Macrae, 29 J. Appl. Bact. 582 (1966); S. Rajan & R. Fox, 39 Soil Sci. Am. Proc. 846 (1973); S. Rajan, 2 Fertilizer Res. 199 (1981). The sulfur oxidizing bacteria produce sulfuric acid that reacts with the mineral phosphate to form monocalcium phosphate (superphosphate), phosphoric acid, and calcium sulfate. While of potential utility, the sulfuric acid pathway is apparently not utilized by most naturally occurring mineral phosphate solubilizing bacteria in the rhizosphere.
The general conclusions of other recent research, A. Moghimi et al., Soil Biol. Biochem. 277 (1978); A. Moghimi et al., 10 Soil Biol. Biochem. 283 (1978); A. Moghimi et al., 10 Soil Biol. Biochem. 289 (1980); N. Subba Rao 7 Interdisciplinary Sci. Rev. 220 (1982); N. Subba Rao, Phosphate Solubilization by Soil Microorganisms, in Advances in Agricultural Microbiology 295 (N. Subba Rao ed., 1982), comport with those of previous studies: (1) a significant percentage of bacteria isolated from soil has the ability to solubilize dicalcium phosphate and/or other poorly soluble mineral phosphates; (2) production of low molecular weight organic acids, which often accompany phosphate solubilization, may account for the phenomenon either through acidification, calcium chelation, or both; and (3) the population of mineral phosphate solubilizing bacteria is substantially higher in the rhizosphere than in non-rhizosphere areas.
Because of the low level of soluble orthophosphate in most ecosystems, bacteria have evolved genetic systems that control the ability to extract orthophosphate from poorly soluble organic and mineral phosphates. The most extensively studied example of the bacterial phenomenon of enhanced capability to solubilize exogenous organic phosphorus and thus increase the external orthophosphate concentration concerns the Phosphate Starvation Inducible (PSI) multicomponent gene system or regulon of E. coli. B. Wanner & R. McSharry, 158 J. Mol. Biol. 347 (1982). Under conditions of phosphorus starvation, this regulon, which includes the gene for bacterial alkaline phosphatase, is induced to hydrolyze organic phosphates to orthophosphate and facilitate its uptake.
The genetic regulation of bacterial mineral phosphate solubilization capability has also been addressed. A. Goldstein, 1 Am. J. Alt. Agric. 51 (1986); A. Goldstein & S. -T. Liu, 5 Bio/Technology 72 (1987). Erwinia herbicola EH010 (ATCC 39368) was shown to have an inducible/repressible mineral phosphate solubilizing trait. Inoculation of agar plates containing insoluble dicalcium phosphate and selected concentrations of soluble phosphate showed that with incremental increases in soluble phosphate there was a corresponding decrease in clearing (solubilization) zone without affecting colony size. Significant repression of gene expression was observed at exogenous soluble phosphate concentrations above 1 mM, and total repression of the trait was evident at concentrations above 20 mM. Further, transformation of E. coli, which is incapable of solubilizing hydroxyapatite, with a cosmid library of E. herbicola genomic DNA resulted in isolation of a recombinant clone that exhibited inducible/repressible hydroxyapatite solubilization at a level comparable with E. herbicola.
Copending U.S. patent application Ser. No. 08/114,410, filed Aug. 30, 1993, and entitled "Genetic and Biochemical Pathway for Bacterial Solubilization of Rock Phosphate," discloses a process for converting rock phosphate ore into soluble phosphate by culturing bacterial cells capable of producing and accumulating gluconic acid in an aqueous medium containing insoluble rock phosphate ore. The gluconic acid acidifies the medium, thus solubilizing the rock phosphate. Gluconic acid is produced by oxidation of glucose catalyzed by a quinoprotein glucose dehydrogenase. S. -T. Liu et al., 174 J. Bacteriol. 5814 (1992).